1. Field of Invention
The present invention relates generally to the field of fluid analysis at temperature and pressure conditions existing at the source of the fluid, or at least temperatures different than ambient, including, but not limited to, reservoir hydrocarbon and aqueous based fluids, drilling muds, frac fluids, and the like having one or more phases (gases, solids and liquids).
2. Related Art
Fluid systems, under the influence of changes in one or more of pressure, temperature, fluid mixing, and/or chemical composition, may contain or develop solid particles that are of interest. One method of investigation uses visible light passing through a sample of the fluid to study the development and properties of these solid particles. There are equipment and experimental limitations to increasing the power of the light source, yet some fluid samples transmit less light than others; therefore it is desirable to be able to change the thickness of the fluid sample under investigation. Furthermore it is desirable to make this change while the sample remains at or near the pressure and temperature of interest.
U.S. Pat. No. 7,079,242, assigned to Core Laboratories, discloses a method and apparatus for determining characteristics of particles in a fluid sample. The method comprises passing light through a window in a view cell in order to observe particles of interest in a pressurized fluid. The operation of pumps causes a sample fluid to pass through the view cell, the view cell having ports connected to a chamber having windows positioned on either side of the chamber to allow visual examination of the sample fluid as it traverses through the chamber. The size of chamber may be adjustable to maintain a predetermined light transmission through the sample fluid as the properties of the fluid samples change. However, adjustment of the size of the sample chamber, either manually or hydraulically, requires rotating the cell windows. This is disadvantageous if one desires to study the fluid using polarized light, as the alignment of the crystal axes of the windows will change upon rotation.
U.S. Pat. No. 5,003,174, assigned to Bruker Analytische Messtechnik GmbH, discloses an optical high-pressure view cell that has stepped windows that are optimized for infrared spectroscopy. By giving the windows of the measuring cell according to the patent a stepped design it is possible to make the distance between surfaces defining the sample chamber as small as desired, by convenient selection of the dimensions of a central, cylindrical neck portion of the cell windows, making it possible to realize chambers with an extremely short light path required for certain measurements. This device, however, requires the cell to be disassembled and reassembled with windows of different central, cylindrical neck portion lengths in order to change the sample chamber size, which is inconvenient and time consuming, and does not discuss use of polarized light.
U.S. Pat. No. 5,905,271, assigned to Wedgewood Technology, discloses an inline optical sensor with vernier optical pathlength adjustment and photometric calibration, wherein the adjustable optical path intersects a flowing product stream. The disclosed device apparently permits the optical pathlength through the product stream to be adjusted with a degree of precision which is more than an order of magnitude greater than tolerances of previous inline optical sensors, and permits calibration standards to be inserted and removed without disturbing the pathlength of the optical system or degrading the signal from the system. While it appears the pathlength can be adjusted in this device without changing alignment of the windows, there is no discussion of use of polarized light or cross-polarization filters; only a spectral filter for light entering a detector, and calibration filter are discussed.
Karan and Ratulowski described visual measurement techniques and apparatus for visually measuring pour point and other properties of waxy crude oils at pressures up to 3000 psi[21 MPa]. Measurements of the live oil wax appearance temperature (WAT) were performed using a high pressure cross polar microscope and a laser-based solids detection system. Karan, K., Ratulowski, J., “Measurement of Waxy Crude Properties Using Novel Laboratory Techniques, SPE 62945 (2000), Society of Petroleum Engineers.
Ratulowski et al., “Flow Assurance and Subsea Productivity: Closing the Loop with Connectivity and Measurements”, SPE Paper 90244 (2004) Society of Petroleum Engineers Inc., describe a dynamic approach to managing the risk of hydrocarbons flow interruptions, including the use of real-time measurements during production form the reservoir, the wellbore, and the subsea infrastructure, in monitoring and optimizing a hydrocarbon production system.
Asphaltenes are heavy, highly aromatic molecules that often precipitate from oils due to reductions in pressure and/or temperature or blending with incompatible fluids (see A. Hammami and J. Ratulowski in: Asphaltenes, Heavy Oils and Petroleomics, Oliver C. Mullins, Eric Y. Sheu, Ahmed Hammami, Alan Marshall, Editors, Kluwer Academic Publications, PRECIPITATION AND DEPOSITION OF ASPHALTENES IN PRODUCTION SYSTEMS: A FLOW ASSURANCE OVERVIEW, Chapter 23, 2006). Asphaktenes also contain multiple polar compounds, and may contain intramolecular species including oxygen, nitrogen, and sulfur, that may make the asphaltene molecules surface active. This surface activity may lead to asphaltene deposition on the walls of process equipment and transportation pipelines and allow asphaltene to participate in the stabilization of water-in-oil emulsions. The “strength” of the surface activity of individual asphaltene molecules is dependent on the variation in asphaltene composition. There is experimental evidence that a small, specific sub-fractions of the asphaltene is responsible for the deposits found on solid surfaces (see, for example, M. Zougari, S. Jacobs, A. Hammami, G. Broze, M. Flannery, J. Ratulowski and A. Stankiewicz, “Novel Organic Solid Deposition and Control Device for Live Oils: Design and Applications” Energy & Fuels, 20 (2006) 1656-1663).
Current experimental methods of studying asphaltenes in reservoir fluids involve the detection of solid precipitates in visual or non-visual pressure-volume-temperature (PVT) cells. A hydrocarbon-based fluid would be placed inside the PVT cell under pressure and temperature conditions similar to those experienced within a petroleum reservoir or in the petroleum production process. The pressure and/or temperature of the fluid would then be changed to induce the formation of a solid precipitate (e.g. asphaltene). Detection of solid formation in the hydrocarbon fluid may be done using near-infrared detectors, x-ray detection, or visual detection via a high pressure, high temperature microscopy method. The devices used in these detection methods are limited to suspended particle detection only and cannot change cell size, nor do they use polarized light for analysis (i.e. they can not distinguish between asphaltenes and waxes). They are also limited to fluids that are transparent enough to allow sufficient light to pass (i.e., they cannot be used with dark oils).
In the case of precipitated asphaltenes, high-temperature, high-pressure filtration may be used to collect the asphaltene aggregates and/or floes. While commonly used, the high-pressure, high-temperature filtration process contains some potentially serious limitations to the analysis of asphaltene precipitate. The first and obvious limitation is that the floc size and amount of the recovered asphaltene depends on the pore size of the filter. Also, one must be very careful not to cause precipitated solids (wax and/or asphaltenes) to grate through the pores of the filter by creating too large of a pressure drop across the filter. Secondly, asphaltenes collected by filtration often contain trapped oil that contains dissolved organic solids (wax or asphaltene). There is a risk that these dissolved solids, particularly asphaltenes, can be precipitated during the removal of the trapped oil. The solids from the trapped oil would then be carried through with the filtered asphaltene precipitate, thus representing a “contaminant” that can affect subsequent analytical characterization. Finally, any measurements completed on the filtered solids (e.g., asphaltene) will only provide information about “average” asphaltene properties. The current protocols do not permit sampling and analysis of individual floes and/or aggregates using adjustable pathlength cells and polarized light, an exercise that may reveal possible variations in chemical composition between the aggregates. With an adjustable pathlength, high-temperature, high-pressure cell and polarized light, it may be possible to detect these compositional variations that contribute to both the surface activity and aggregation behavior of asphaltenes.
A long, but heretofore unmet, need exists in the art for apparatus and methods for studying properties of a sample at temperatures and pressures representing those existing at the source of the sample, or at least different than ambient laboratory conditions, using an adjustable pathlength view cell and polarized light.